A Betta, 4 tetras, cory and upsidedown catfish is overstocked. The upsidedown catfish can get to be 4"..most corys get to be at least 2(although I don't know what size you have) neons are small but I always count them at an inch-ish..even without the Betta you're kinda full.

Both corys and UDCatfish are social little guys and prefer to be in groups which you definitely don't have room for.
If you rehomed the tetras and the UDCatfish, you could get a few more corys and keep the betta in.

So, no..you can't add anything else. In fact, if I understand correctly, you have a betta in there already and have a divider?
Thats too much. Get the Betta his own 2G tank and definitely don't get another one for that 10G until you rehome some of your existing fish.

If your 10G is healthy and happy with the tetras, cory and UDCatfish, I'd leave it. A Betta just won't fit in there.
If you wanted to take them to your local fish store, you would probably get a credit...then....
You could divide the 10G into two or three and have more room for Betta boys!

If your 10G is healthy and happy with the tetras, cory and UDCatfish, I'd leave it. A Betta just won't fit in there.
If you wanted to take them to your local fish store, you would probably get a credit...then....
You could divide the 10G into two or three and have more room for Betta boys!

There is also a betta in there with a divider. But even without the betta, the set up just doesn't work.

i have plenty of space trust me this is what it looks like. P.S. can any one answer my real question? you know the one about can a betta be in a tank with the fish i have with out killing them?

Yes they can be with those fish but it has to be in a tank much larger. And no, you don't have plenty of space. Sorry. Cories need to be in groups of 5 or more, so that single cory will probably get stressed and sick from lack of companionship. In your current set up the fish do not have the amount of room the need.